The nexon website defines NX as a , "virtual currency used throughout all games serviced by Nexon America Inc. NX can be spent on in-game services such as the Cash Shop in MapleStory to buy new items, accessories, and add new features. There are two types of NX depending on how you charge it."

Also, "a great way to enhance your gaming experience with the many offerings available in our Cash Shop. Purchase NX or surprise someone special with a Gift today!"
Rough translation:

NX: A virtual currency used to buy pixels. A source of income for Nexon and a way allowing noobs to buy skill. [think they have some]